The present invention relates to a pattern inspection technique for checking patterns on objects to be inspected by comparing images.
The conventional pattern inspecting technique has been generally used such that, when an image to be inspected and a reference image are compared with each other in order for defects to be detected, the edges of the patterns of both images are aligned so that the minimum displacement between the edges can be computed as a piece of information for matching (alignment). The proposed means used for this purpose are normalized cross-correlation, and summation of residual errors.
Also, examples of the pattern inspection technique by comparing images are disclosed in JP-A-05-264467 and JP-A-2001-22935. The JP-A-05-264467 describes that an object to be inspected on which repeated patterns are regularly arranged is sequentially scanned by a line sensor, and each of the images picked up by the sensor and that image after being delayed by the pitch of the repeated patterns are compared to produce an amount of positional deviation, or displacement, and both successive imaged are aligned on the basis of the displacement to produce a difference, in which case if the difference is greater than a threshold, this object is decided to have a defect, while if it is smaller, the object is decided not to be defective.
The JP-A-2001-22935 describes that, in order that even an object to be inspected on which various patterns are formed mixed can be checked with the error due to mismatching being reduced, a candidate position for matching (alignment) is determined with respect to each of the images successively fed, and alignment is made after determining the optimum matching position of each image from the collection of a plurality of the matching candidate points.
When images of, for example, FIGS. 2A, 2B and 2C are checked, the conventional alignment-type inspection method described in JP-A-05-264467 cannot uniquely determine the matching position, and thus has the possibility of often causing computation error of displacement. FIG. 2A shows the case of extremely small edge information, or when the ratio of edge portion to the whole region for alignment (hereafter, referred to as pattern density) is small. FIG. 2B is the case when there are an extremely large number of edges in a particular direction, and FIG. 2C is the case when there are an extremely large number of small-pitch patterns.
Moreover, if other normal patterns exist as a small part in addition to many patterns like those shown in FIGS. 2A, 2B and 2C, has the possibility of causing nuisance defect (erroneously detected information) in that part. In addition, since matching candidate positions for images to be matched are calculated by using all regions, an enormous amount of data is used for the computation, thus taking a long processing time. For high speed, it is necessary to construct the Misalignment detection part as hardware or to increase the number of arithmetic circuits, thus making it difficult to reduce the cost and size of apparatus.
The technique described in JP-A-2001-22935 also uses a huge amount of data for the matching candidate points to be computed by using all regions, thus making it difficult to increase the processing speed, and hence to reduce the cost and size of apparatus as in the JP-A-05-264467.
Accordingly, it is an object of the invention when the images are compared for pattern inspection, to make it possible (1) to accurately detect particle and pattern defects, (2) to reduce the amount of computation for misalignment detector of images, thus increasing the processing speed, (3) to precisely detect the amount of displacement between images, and (4) to reduce the cost and size of apparatus.